Phase characterization of diffusion soldered Ni/Al/Ni interconnections

The formation and growth of intermetallic phases in the Ni-Al system during a novel joining process for Ni/Ni interconnections based on diffusion soldering has been studied. The Ni/Al/Ni bonds were accomplished by isothermal solidification and subsequent interdiffusion of Ni and Al in the Ni/Al/Ni joints held at a temperature of 720degreesC. Optical and scanning electron microscopy, electron probe microanalysis and X-ray diffraction analysis were used to characterize the microstructural changes as a function of the reaction time. The following phases appeared sequentially: liquid Al --> Al3Ni --> Al3Ni2 --> AlNi (stoichiometric) --> AlNi (Ni-rich) --> AlNi3. At intermediate stages two to four phases coexisted. The NiAl phase occurred in two variants, namely a Ni-rich AlNi (60 at.% Ni) and stoichiometric AlNi. The joining process was completed after 30 h of reaction. Then only AlNi3 was present in the Ni/Al/Ni interconnection zone. The quality of the resultant bond and the high melting point of the AlNi3 phase (1360degreesC) indicate a great potential of the diffusion soldering for the joining of heat dissipating devices used in electronics and electrotechnics.